1. Technical Field
The present disclosure relates generally to current control mechanisms for use in low power consumption circuits.
2. Background
The trend of reduced power consumption in circuits has led to the increased use of lower supply voltages. Consistent with this trend is the use of 3.3-volt power supplies, which are significantly lower than conventional 5.0-volt power supplies, and which impose a new set of restrictions that must be taken into account. For instance, while InGaP HBT (indium gallium phosphide heterojunction bipolar transistor) devices are particularly well-suited for optoelectronic device driver applications such that illustrated in FIG. 1, the InGaP HBT base-to-emitter voltage (Vbe) of about 1.4 volts is appreciably higher than that of conventional HBT and BJT (bipolar junction transistors) devices fabricated in Si/Ge or InP. This makes it very challenging to design current sources (current mirrors) for InGaP HBT circuits because two Vbe drops of InGaP HBT devices take up about 2.8 volts, leaving only about 500 mV headroom for the current source. The 500 mV remaining headroom for the Vce (collector-to-emitter voltage) of the current mirror is not sufficient as the current mirror transistor is close to saturation. In addition, when the Vcc (power supply voltage) drops due to some variations, for instance by 5%, the voltage headroom drops further, to for example only 350 mV.
The driver circuit 100 of FIG. 1 employs an IC (integrated circuit) 102 to drive a modulator 104 in order to modulate a light output signal. IC 102 relies on transistors, such as HBTs or BJTs, to effect the necessary voltage switching at the high frequencies required for fast light modulation operations. Inverted and non-inverted input signals are provided at input side 106. The output voltage of the IC needs to be flexible because different modulators require different output voltage from the IC in order to switch the light on and off. The ability to change the current is also important to take process variation in die fabrication into account. This output voltage change is realized by changing the current in the output stage of the driver IC. Power, typically about 5.0 volts, is provided at terminal Vcc. The use of lower power levels, such as 3.3 volts, particularly with InGaP HBT devices, can be problematic due to the limited headroom imposed by the increased (1.4 volts) base-to-emitter voltage Vbe of current mirrors striving to employ such devices. There is therefore a long-felt need for a current source other than a conventional current mirror to drive a low power consumption optoelectronic driver circuit using InGaP HBT devices.